Many machines drive one or more loads by hydraulic force. Common examples of such machines include earth handling machines such as front end loaders, backhoes and the like. In such machines, a load, such as a scoop or shovel, is supported vertically (or controlled while moving in another direction) by the force exerted by confined hydraulic fluid. The uncontrolled escape of such fluid, as when a hose bursts, would allow the load to drop (or move uncontrolled in the other direction) absent protective measures.
Many of the previously known protective measures employ a check valve located at the hydraulic actuator, such as a piston in a cylinder, which is powering the load. The check valve prevents the escape of the fluid from the cylinder in the event a hose breaks downstream from the cylinder. This prevents the load from moving uncontrollably but leaves it hung up and beyond further control by the operator.
There is a need for a relatively simple and economical means both to prevent uncontrolled movement by a load in the event of such a fluid loss and to allow the load to be moved under the control of the operator.